Living organisms have evolved a complex array of biochemical pathways that enable them to recognize and respond to signals from the environment. These pathways include receptor organs, hormones, second messengers, and enzymatic modifications. At present, little is known about the signal transduction pathways that are activated during a plant's response to attack by a pathogen, although this knowledge is central to an understanding of disease susceptibility and resistance. A common form of plant resistance is the restriction of pathogen proliferation to a small zone surrounding the site of infection. In many cases, this restriction is accompanied by localized death (i.e., necrosis) of host tissues. Together, pathogen restriction and local tissue necrosis characterize the hypersensitive response. In addition to local defense responses, many plants respond to infection by activating defenses in uninfected parts of the plant. As a result, the entire plant is more resistant to a secondary infection. This systemic acquired resistance can persist for several weeks or more (R. E. F. Matthews, Plant Virology (Academic Press, New York, ed. 2, 1981)) and often confers cross-resistance to unrelated pathogens (J. Kuc, in Innovative Approaches to Plant Disease Control, I. Chet, Ed. (Wiley, New York, 1987), pp. 255-274, which is hereby incorporated by reference).
Expression of systemic acquired resistance is associated with the failure of normally virulent pathogens to ingress the immunized tissue (Kuc, J., "Induced Immunity to Plant Disease," Bioscience, 32:854-856 (1982), which is hereby incorporated by reference). Establishment of systemic acquired resistance is correlated with systemic increases in cell wall hydroxyproline levels and peroxidase activity (Smith, J. A., et al., "Comparative Study of Acidic Peroxidases Associated with Induced Resistance in Cucumber, Muskmelon and Watermelon," Physiol. Mol. Plant Pathol. 14:329-338 (1988), which is hereby incorporated by reference) and with the expression of a set of nine families of so-called systemic acquired resistance gene (Ward, E. R., et al., "Coordinate Gene Activity in Response to Agents that Induce Systemic Acquired Resistance," Plant Cell 3:49-59 (1991), which is hereby incorporated by reference). Five of these defense gene families encode pathogenesis-related proteins whose physiological functions have not been established. However, some of these proteins have antifungal activity in vitro (Bol, J. F., et al., "Plant Pathogenesis-Related Proteins Induced by Virus Infection," Ann. Rev. Phytopathol. 28:113-38 (1990), which is hereby incorporated by reference) and the constitutive expression of a bean chitinase gene in transgenic tobacco protects against infection by the fungus Rhizoctonia solani (Broglie, K., et al., "Transgenic Plants with Enhanced Resistance to the Fungal Pathogen Rhizoctonia Solani," Science 30 254:1194-1197 (1991), which is hereby incorporated by reference), suggesting that these systemic acquired resistance proteins may contribute to the immunized state (Uknes, S., et al., "Acquired Resistance in Arabidopsis," Plant Cell 4:645-656 (1992), which is hereby incorporated by reference).
Salicylic acid appears to play a signal function in the induction of systemic acquired resistance since endogenous levels increase after immunization (Malamy, J., et al., "Salicylic Acid: A Likely Endogenous Signal in the Resistance Response of Tobacco to Viral Infection," Science 250:1002-1004 (1990), which is hereby incorporated by reference) and exogenous salicylate induces systemic acquired resistance genes (Yalpani, N., et al., "Salicylic Acid is a Systemic Signal and an Inducer of Pathogenesis-Related Proteins in Virus-Infected Tobacco," Plant Cell 3:809-818 (1991), which is hereby incorporated by reference), and acquired resistance (Uknes, S., et al., "Acquired Resistance in Arabidopsis," Plant Cell 4:645-656 (1992), which is hereby incorporated by reference). Moreover, transgenic tobacco plants in which salicylate is destroyed by the action of a bacterial transgene encoding salicylate hydroxylase do not exhibit systemic acquired resistance (Gaffney, T., et al., "Requirement of Salicylic Acid for the Induction of Systemic Acquired Resistance," Science 261:754-296 (1993), which is hereby incorporated by reference). However, this effect may reflect inhibition of a local rather than a systemic signal function, and detailed kinetic analysis of signal transmission in cucumber suggests that salicylate may not be essential for long-distance signaling (Rasmussen, J. B., et al., "Systemic Induction of Salicylic Acid Accumulation in Cucumber after Inoculation with Pseudomonas Syringae pv. Syringae," Plant Physiol. 97:1342-1347) (1991), which is hereby incorporated by reference).
Immunization using biotic agents has been extensively studied. Green beans were systemically immunized against disease caused by cultivar-pathogenic races of Colletotrichum lindemuthianum by prior infection with either cultivar-nonpathogenic races (Rahe, J. E., "Induced Resistance in Phaseolus Vulgaris to Bean Anthracnose," Phytopathology 59:1641-5 (1969); Elliston, J., et al., "Induced Resistance to Anthracnose at a Distance from the Site of the Inducing Interaction," Phytopathology 61:1110-12 (1971); Skipp, R., et al., "Studies on Cross Protection in the Anthracnose Disease of Bean," Physiological Plant Pathology 3:299-313 (1973), which are hereby incorporated by reference), cultivar-pathogenic races attenuated by heat in host tissue prior to symptom appearance (Rahe, J. E., et al., "Metabolic Nature of the Infection-Limiting Effect of Heat on Bean Anthracnose," Phytopathology 60:1005-9 (1970), which is hereby incorporated by reference) or nonpathogens of bean. The anthracnose pathogen of cucumber, Colletotrichum lagenarium, was equally effective as non-pathogenic races as an inducer of systemic protection against all races of bean anthracnose. Protection was induced by C. lagenarium in cultivars resistant to one or more races of C. lindemuthianum as well as in cultivars susceptible to all reported races of the fungus and which accordingly had been referred to as `lacking genetic resistance` to the pathogen (Elliston, J., et al., "Protection of Bean Against Anthracnose by Colletotrichum Species Nonpathogenic on Bean," Phytopathologische Zeitschrift 86:117-26 (1976); Elliston, J., et al., "A Comparative Study on the Development of Compatible, Incompatible and Induced Incompatible Interactions Between Collectotrichum Species and Phaseolus Vulgaris," Phytopathologische Zeitschrift 87:289-303 (1976), which are hereby incorporated by reference). These results suggest that the same mechanisms may be induced in cultivars reported as `possessing` or `lacking` resistance genes (Elliston, J., et al., "Relation of Phytoalexin Accumulation to Local and Systemic Protection of Bean Against Anthracnose," Phytopathologische Zeitschrift 88:114-30 (1977), which is hereby incorporated by reference). It also is apparent that cultivars susceptible to all races of C. lindemuthianum do not lack genes for resistance mechanisms against the pathogen.
Kuc, J., et al., "Protection of Cucumber Against Collectotrichum Lagenarium by Colletotrichum lagenarium," Physiological Plant Pathology 7:195-9 (1975), which is hereby incorporated by reference), showed that cucumber plants could be systemically protected against disease caused by Colletotrichum lagenarium by prior inoculation of the cotyledons or the first true leaf with the same fungus. Subsequently, cucumbers have been systemically protected against fungal, bacterial, and viral diseases by prior localized infection with either fungi, bacteria, or viruses (Hammerschmidt, R., et al., "Protection of Cucumbers Against Colletotrichum Lagenarium and Cladosporium Cucumerinum," Phytopathology 66:790-3 (1976); Jenns, A. E., et al., "Localized Infection with Tobacco Necrosis Virus Protects Cucumber Against Colletotrichum Lagenarium," Physiological Plant Pathology 11:207-12 (1977); Caruso, F. L., et al. "Induced Resistance of Cucumber to Anthracnose and Angular Leaf Spot by Pseudomonas Lachrymans and Colletotrichum Lagenarium," Physiological Plant Pathology 14:191-201 (1979); Staub, T., et al., "Systemic Protection of Cucumber Plants Against Disease Caused by Cladosporium Cucumerinum and Colletotrichum Lagenarium by Prior Localized Infection with Either Fungus," Physiological Plant Pathology, 17:389-93 (1980); Bergstrom, G. C., et al., "Effects of Local Infection of Cucumber by Colletotrichum Lagenarium, Pseudomonas Lachrymans or Tobacco Necrosis Virus on Systemic Resistance to Cucumber Mosaic Virus," Phytopathology 72:922-6 (1982); Gessler, C., et al., "Induction of Resistance to Fusarium Wilt in Cucumber by Root and Foliar Pathogens," Phytopathology 72:1439-41 (1982); Basham, B., et al., "Tobacco Necrosis Virus Induces Systemic Resistance in Cucumbers Against Sphaerotheca Fuliginea," Physiological Plant Pathology 23:137-44 (1983), which are hereby incorporated by reference). Non-specific protection induced by infection with C. lagenarium or tobacco necrosis virus was effective against at least 13 pathogens, including obligatory and facultative parasitic fungi, local lesion and systemic viruses, wilt fungi, and bacteria. Similarly, protection was induced by and was also effective against root pathogens. Other curcurbits, including watermelon and muskmelon have been systemically protected against C. lagenarium (Caruso, F. L., et al., "Protection of Watermelon and Muskmelon Against Colletotrichum Lagenarium by Colletotrichum Lagenarium," Phytopathology 67:1285-9 (1977), which is hereby incorporated by reference).
Systemic protection in tobacco has also been induced against a wide variety of diseases (Kuc, J., et al., "Immunization for Disease Resistance in Tobacco," Recent Advances in Tobacco Science 9:179-213 (1983), which is hereby incorporated by reference). Necrotic lesions caused by tobacco mosaic virus enhanced resistance in the upper leaves to disease caused by the virus (Ross, A. F., et al., "Systemic Acquired Resistance Induced by Localized Virus Infections in Plants," Virology 14:340-58 (1961); Ross, A. F., et al., "Systemic Effects of Local Lesion Formation," In: Viruses of Plants pp. 127-50 (1966), which are hereby incorporated by reference). Phytophthora parasitica var. nicotianae, P. tabacina and Pseudomonas tabaci and reduced reproduction of the aphid Myzus persicae (McIntyre, J. L., et al., "Induction of Localized and Systemic Protection Against Phytophthora Parasitica var. nicotianae by Tobacco Mosaic Virus Infection of Tobacco Hypersensitive to the Virus," Physiological Plant Pathology 15:321-30 (1979); McIntyre, J. L., et al., "Effects of Localized Infections of Nicotiana Tabacum by Tobacco Mosaic Virus on Systemic Resistance Against Diverse Pathogens and an Insect," Phytopathology 71:297-301 (1981), which are hereby incorporated by reference). Infiltration of heat-killed P. tabaci (Lovrekovich, L., et al., "Induced Reaction Against Wildfire Disease in Tobacco Leaves Treated with Heat-Killed Bacteria," Nature 205:823-4 (1965), which is hereby incorporated by reference), and Pseudomonas solanacearum (Sequeira, L, et al., "Interaction of Bacteria and Host Cell Walls: Its Relation to Mechanisms of Induced Resistance," Physiological Plant Pathology 10:43-50 (1977), which are hereby incorporated by reference), into tobacco leaves induced resistance against the same bacteria used for infiltration. Tobacco plants were also protected by the nematode Pratylenchus penetrans against P. parasitica var. nicotiana (McIntyre, J. L., et al. "Protection of Tobacco Against Phytophthora Parasitica Var. Nicotianae by Cultivar-Nonpathogenic Races, Cell-Free Sonicates and Pratylenchus Penetrans," Phytopathology 68:235-9 (1978), which is hereby incorporated by reference).
Cruikshank, I. A. M., et al., "The Effect of Stem Infestation of Tobacco with Peronospora Tabacina Adam on Foliage Reaction to Blue Mould," Journal of the Australian Institute of Agricultural Science 26:369-72 (1960), which is hereby incorporated by reference, were the first to report immunization of tobacco foliage against blue mould (i.e., P. tabacina) by stem injection with the fungus, which also involved dwarfing and premature senescence. It was recently discovered that injection external to the xylem not only alleviated stunting but also promoted growth and development. Immunized tobacco plants, in both glasshouse and field experiments, were approximately 40% taller, had a 40% increase in dry weight, 30% increase in fresh weight, and 4-6 more leaves than control plants (Tuzun, S., et al., "The Effect of Stem Injections with Peronospora Tabacina and Metalaxyl Treatment on Growth of Tobacco and Protection Against Blue Mould in the Field," Phytopathology 74:804 (1984), which is hereby incorporated by reference). These plants flowered approximately 2-3 weeks earlier than control plants (Tuzun, S., et al., "Movement of a Factor in Tobacco Infected with Peronospora Tabacina Adam which Systemically Protects Against Blue Mould," Physiological Plant Pathology 26:321-30 (1985), which is hereby incorporated by reference).
Systemic protection does not confer absolute immunity against infection, but reduces the severity of the disease and delays symptom development. Lesion number, lesion size, and extent of sporulation of fungal pathogens are all decreased. The diseased area may be reduced by more than 90%.
When cucumbers were given a `booster` inoculation 3-6 weeks after the initial inoculation, immunization induced by C. lagenarium lasted through flowering and fruiting (Kuc, J., et al., "Aspects of the Protection of Cucumber Against Colletotrichum Lagenarium by Colletotrichum Lagenarium," Phytopathology 67:533-6 (1977), which is hereby incorporated by reference). Protection could not be induced once plants had set fruit. Tobacco plants were immunized for the growing season by stem injection with sporangia of P. tabacina. However, to prevent systemic blue mould development, this technique was only effective when the plants were above 20 cm in height.
Removal of the inducer leaf from immunized cucumber plants did not reduce the level of immunization of pre-existing expanded leaves. However, leaves which subsequently emerged from the apical bud were progressively less protected than their predecessors (Dean, R. A., et al., "Induced Systemic Protection in Cucumber: Time of Production and Movement of the `Signal`," Phytopathology 76:966-70 (1986), which is hereby incorporated by reference). Similar results were reported by Ross, A. F., "Systemic Effects of Local Lesion Formation," In: Viruses of Plants pp. 127-50 (1966), which is hereby incorporated by reference, with tobacco (local lesion host) immunized against tobacco mosaic virus by prior infection with tobacco mosaic virus. In contrast, new leaves which emerged from scions excised from tobacco plants immunized by stem-injection with P. tabacina were highly protected (Tuzun, S., et al., "Transfer of Induced Resistance in Tobacco to Blue Mould (Peronospora Tabacina Adam.) Via Callus," Phytopathology 75:1304 (1985), which is hereby incorporated by reference). Plants regenerated via tissue culture from leaves of immunized plants showed a significant reduction in blue mould compared to plants regenerated from leaves of non-immunized parents. Young regenerants only showed reduced sporulation. As plants aged, both lesion development and sporulation were reduced. Other investigators, however, did not reach the same conclusion, although a significant reduction in sporulation in one experiment was reported (Lucas, J. A., et al., "Nontransmissibility to Regenerants from Protected Tobacco Explants of Induced Resistance to Peronospora Hyoscyami," Phytopathology 75:1222-5 (1985), which is hereby incorporated by reference).
Protection of cucumber and watermelon is effective in the glasshouse and in the field (Caruso, F. L., et al., "Field Protection of Cucumber Against Colletotrichum Lagenarium by C. Lagenarium," Phytopathology 67:1290-2 (1977), which is hereby incorporated by reference). In one trial, the total lesion area of C. lagenarium on protected cucumber was less than 2% of the lesion areas on unprotected control plants. Similarly, only 1 of 66 protected, challenged plants died, whereas 47 of 69 unprotected, challenged watermelons died. In extensive field trials in Kentucky and Puerto Rico, stem injection of tobacco with sporangia of P. tabacina was at least as effective in controlling blue mould as the best fungicide, metalaxyl. Plants were protected 95-99%, based on the necrotic area and degree of sporulation, leading to a yield increase of 10-25% in cured tobacco.
Induced resistance against bacteria and viruses appears to be expressed as suppression of disease symptoms or pathogen multiplication or both (Caruso, F. L., et al., "Induced Resistance of Cucumber to Anthracnose and Angular Leaf Spot by Pseudomonas Lachrymans and Colletotrichum Lagenarium," Physiological Plant Pathology 14:191-201 (1979); Doss, M., et al., "Systemic Acquired Resistance of Cucumber to Pseudomonas Lachrymans as Expressed in Suppression of Symptoms, but not in Multiplication of Bacteria," Acta Phytopathologia Academiae Scientiarum Hungaricae 16: (3-4), 269-72 (1981); Jenns, A. E., et al., "Non-Specific Resistance to Pathogens Induced Systemically by Local Infection of Cucumber with Tobacco Necrosis Virus, Colletotrichum Lagenarium or Pseudomonas Lachrymans," Phytopathologia Mediterranea 18:129-34 (1979), which are hereby incorporated by reference).
As described above, research concerning systemic acquired resistance involves infecting plants with infectious pathogens. Although studies in this area are useful in understanding how systemic acquired resistance works, eliciting such resistance with infectious agents is not commercially useful, because such plant-pathogen contact can weaken or kill plants. The present invention is directed to overcoming this deficiency.